How to "expand" sub array in array of objects

Question

This is my array of objects

[{
    "key1": "value1",
    "key2": "value2",
    "key3": ["value3", "value4"]
}]

The result should be like this

[{
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}, {
    "key1": "value1",
    "key2": "value2",
    "key3": "value4"
}]

So I want to get rid of the sub array in property key3 and get the new equivalent structure, copying all the other properties.

For reasons I cannot change I am supposed to use lodash, but only in version 2.4.2

EDIT: To be more elaborate: I am using a JSON based form engine which allows to use existing functions (like lodash functions) but doesn't allow to define new functions. I also cannot use control structures like for loops. Essentially I can only use chained basic function calls including lodash.

I tried to use map, but map cannot extend an array, it can only convert one array element into something different

Is there any lodash magic I can use here?

EDIT2: Here is an example about what I mean when I say "I cannot introduce new functions". It will check if an array of objects is unique regarding a certain subset of properties

model = [{
    "key1": "value1",
    "key2": "value2",
    "key3": "valuex"
},{
    "key1": "value1",
    "key2": "value2",
    "key3": "valuey"
}]

// will give false because the two objects are not unique regarding the combination of "key1" and "key2"
_.uniq(model.map(_.partialRight(_.pick, ["key1", "key2"])).map(JSON.stringify)).length === model.length

Answer 1

Well, this was a challenge! I have a working solution that covers all the cases I can think of, but please let me know if there is a situation I missed.

---

My general approach started from the end, I knew I was going to use _.zipObject to create the result objects. From there, it was just a matter of making the other properties line up with the necessary key3 values. To do so, I simply copy the property values so each value of key3 has its own copy. Next, I link them back up and create the objects. Finally, I filter out any unnecessary copies of the objects.

NOTE: This approach will not work correctly for an undefined element in key3. I considered this to be an unlikely situation, thus did not attempt to address.

---

The understandable version:

const objects = [{
    "key1": "value1",
    "key2": "value2",
    "key3": ["value3", "value4"]
},
{
    "key1": "value5",
    "key2": "value6",
    "key3": ["value7"]
}];

// Get other key names
const otherKeys = _.without(_.keys(objects[0]), "key3");
// Get values without key3
const otherValues = _.map(_.map(objects, _.partialRight(_.omit, "key3")), _.values);
// Get just key3 values
const onlyKey3 = _.map(objects, "key3");

// Generate dummy range of needed length
const maxLengthKey3 = _.max(_.map(onlyKey3, "length"));
const dummyRange = _.range(maxLengthKey3);

// Grow all arrays to needed length
const newOtherValues = _.flatten(_.map(dummyRange, _.partial(_.identity, otherValues)), true);
const newKey3 = _.flatten(_.map(dummyRange, _.partial(_.map, onlyKey3)));

const pairedValues = _.map(_.zip(newOtherValues, newKey3), _.flatten);
const resultObjects = _.map(pairedValues, _.partial(_.zipObject, _.union(otherKeys, ["key3"])));

// Filter out unnecessary objects
const result = _.filter(resultObjects, "key3");

All in one line:

const objects = [{
    "key1": "value1",
    "key2": "value2",
    "key3": ["value3", "value4"]
},
{
    "key1": "value5",
    "key2": "value6",
    "key3": ["value7"]
}];
// One line
const result = _.filter(_.map(_.map(_.zip(_.flatten(_.map(_.range(_.max(_.map(_.map(objects, "key3"), "length"))), _.partial(_.identity, _.map(_.map(objects, _.partialRight(_.omit, "key3")), _.values))), true), _.flatten(_.map(_.range(_.max(_.map(_.map(objects, "key3"), "length"))), _.partial(_.map, _.map(objects, "key3"))))), _.flatten), _.partial(_.zipObject, _.union(_.without(_.keys(objects[0]), "key3"), ["key3"]))), "key3");

---

Performance:

I expect it to be terrible for a large initial array, or for a large length key3. I especially shudder at the single line version. If anyone complains, I'd make the point that this is caused by the limitations of the execution environment.

---

This was tested in the browser via https://npm.runkit.com/lodash, using var _ = require('[email protected]');

Answer 2

let obj = {
    key1: "value1",
    key2: "value2",
    key3: ["value3", "value4"]
}

let tracker = new Array(obj.key3.length)

let newObjArr = []

for (let i = 0; i < tracker.length; i++) {
  newObjArr.push({
    key1: obj.key1,
    key2: obj.key2,
    key3: obj.key3[i]
  })
}

console.log(newObjArr)